A variety of systems are currently available for providing centralized messaging services, and the services offered are becoming increasingly popular. Such services include voice mail, store and forward type facsimile (FAX) mail, E-mail and the like. Particularly when offered by a local telephone company (TELCO) or through a TELCO's end office switch, these services involve forwarding unanswered calls directed to the subscriber's equipment to a centralized messaging system. The caller then transfers one or more messages to the centralized messaging system for storage in the called subscriber's mailbox. Subsequently, the subscriber calls in to the centralized messaging system to retrieve the messages.
Consider a voice mail service, offered by a local TELCO, as one specific example of a centralized messaging service. With a typical voice mail service offered through local telephone companies, a caller dials the subscriber's normal telephone number. If the subscriber's line identified by the dialed telephone number is busy or if there is no answer, the public switched telephone network forwards the call to a voice mail system. This voice mail system answers the call and provides an answering prompt message to the caller. In many systems, this prompt will include a personalized greeting message recorded by the called subscriber. After the prompt, the voice mail system records a message from the caller in the called subscriber's mailbox.
In many of the voice mail systems, a subscriber retrieves messages from a remote location by calling the subscriber's own telephone number. Assuming that the subscriber's line is not busy since the subscriber is away from home, the call will go unanswered. After a set ringing interval or a predetermined number of rings, the public telephone network will forward the unanswered call to the voice mail system, in the same manner as for any other call directed to the subscriber's telephone number. The voice mail system will provide the normal answering prompt to the subscriber, but the subscriber will respond by inputting some form of command code, for example by actuating one or more specific keys of a dual tone multi-frequency (DTMF) telephone. The voice mail system detects DTMF signals representing the code, executes some form of security procedure (e.g. requiring input of a personal identification code or password) and allows the subscriber access to all of the available mailbox control functions, including message retrieval. The voice mail system may provide a prompt message informing the subscriber of how many messages are stored in the subscriber's mailbox and/or how many messages are new (stored but not yet replayed to the subscriber). The subscriber can then actuate the keys on the telephone to instruct the voice mail system to replay selected stored messages, delete messages, forward messages to other voice mail subscribers, etc.
Retrieval calls of the type described above often involve a long distance telephone call from the subscriber's current location to the voice mail system and/or charges for use of a pay telephone. When there are no new messages stored in the subscriber's mailbox, the subscriber still must complete the call to the voice mail system to determine such a current empty status of the mailbox. As a result, the subscriber incurs long distance charges and/or pay telephone charges but receives no new messages. These problems also arise in calling in to check the status of mailboxes in other types of centralized messaging systems, such as FAX mail systems or E-mail systems.
A similar problem has been recognized in the answering machine art, where a subscriber calls home to retrieve messages. An advanced answering machine will wait for a relatively long ringing interval before answering (e.g. six rings) if the machine has stored no new messages since the last retrieval operation. However, if the answering machine has stored one or more new messages since the last retrieval operation, the machine waits for a shorter interval before answering (e.g. two rings). Consequently, the owner calling in to check if messages are recorded can count the number of ringback signals heard. If the owner hears three or four ringbacks, then the machine has stored no new messages, and the owner can hang up before the machine answers the call. Consequently, the owner incurs no toll or pay telephone charges for the call. If the machine has stored new messages, the machine will answer incoming calls after a short interval. If the owner is calling in, this quick answer will occur before the owner might choose to hang up, and the owner can retrieve the new messages.
A solution similar to that used in answering machines apparently has not been applied to centralized messaging systems. To do so would require the centralized answering system to identify the called subscriber prior to answering each incoming call and then vary the number of rings before answering based on the status of the called subscriber's mailbox. This would complicate operations of the centralized messaging systems. Typically, unanswered calls are forwarded to the centralized messaging system after a set number of rings at the called station. Thus, the caller has already waited for some measurable interval. To force all callers to wait an additional long number of rings before the messaging system answered the call (when no new messages are already stored in the called subscriber's mailbox), would discourage many callers. Some such callers might choose to hang up before the messaging system answered their calls, not even knowing that they could leave a message. The resultant failures to store messages for the subscriber would defeat the purpose of the centralized messaging service.
Also, many messaging systems vary the number of rings before answering in order to queue incoming calls somewhat during peak usage. If call processing resources are heavily loaded or not available, the system may allow the new incoming call to go unanswered until some resource becomes available to process the call. Systems varying the ringing period before answering for one purpose cannot effectively vary that period for another newer purpose. If the messaging system varies the ringing period to queue calls, the system can not also vary that period as an indication of mailbox status; and if the centralized messaging system were to vary the ringing period as an indication of mailbox status, that system can no longer vary that period to queue calls.
From the above discussion it should be clear that a need exists to provide some indication of the status of a subscriber's mailbox or other service provided by a centralized messaging system to a calling subscriber, before the centralized messaging system answers an incoming call.